Here's my last foray into this thread...then I'm done. I hate bumping it as it is. I'll even make this a constructive post, and end the hate.
Alot of this depends on what your definition of "farm" is. It it takes over a minute to kill an elite pack....it's not farming for me. It's a coma. Can you survive and kill enemies with 20M at 2.31AS....sure. With Ziss's gear and build sets. Sure. But its not "efficient". You're going to die, probably quite a bit, and get tired of it....and not freezing well enough. But if you're really good micro'ing situations, and be patient, you can start to learn to avoid some deaths...and it will work.
What MP to farm partly depends on what you find enjoyable and how much you involve the math!
Sure, you get a 250% bonus to MP at MP10, which is the largest bonus that you can get. Many people therefore assume that its the best place to farm. Untrue.
Ex1 - Say you have 75% MF gear/paragon, 75% from your stacks, and 250% from MP10...that makes 400% total. Say also that it takes you 1 hr to do an Act3 run - maybe all the keeps, Fields of Slaughter, Bridge, Craters, Key warden, etc.
Ex2 - Say you have 75% MF gear/paragon, 75% from your stacks, and 200% from MP8...that makes 350% total. Since the enemies have half the amount of life, lets say that you clear it in about 30 minutes.
Overall....it makes little sense to farm at MP10!!! You could farm MP8 TWICE in the time that it takes you to farm MP10 once....and only for an extra 50% MF at MP10?!. Another way of looking at it is that by doing MP8 twice in 1 hour, you have an effective MF of 700% (versus farming MP10 once at 400% in one hour)!!!
So....I just go for what "feels" efficient....killing an elite pack in 20 seconds...something like that (for me, that's MP7-8). For awhile I did low MP speed farming.....you can breeze through a run in 10-15 min on MP3 (roughly 75+75+75=225%...if you do it 4x an hour, that'd be an effective ~800% MF) - so, that's another possibility.
Now, I'm playing a little fast an lose with the numbers, so don't definitively hold me to them. But this is generally how MF works. Don't get caught up in the MP10 farming hype (if you can't kill an elite in ~20 sec or so)....its too slow to make the extra MF valuable. Instead, farm lower MP's more effectively and efficiently.
BTW, I'd suggest a DPS of, like, 200k to farm MP10 "efficiently." Like I said....with ~140k DPS I typically farm MP7 or 8. It makes little sense to go up higher. I can make it though a whole run with few, if any, deaths. A death means retracing empty parts of maps, and is a waste. If you're "undergrared" with a 20M build and die alot...then that's also making for an inefficient run.
I have the mitigation to hang in MP10,....but the DPS isn't there. And it's certainly not there with any 20M build.
Edited by PieHole#1628 on 3/19/2013 2:02 PM PDT